1. Field of the Invention
The present invention relates to an electronic circuit for automatically identifying a carrier frequency of an audio signal, that has been separated from a transmitted television signal, from a number of frequencies that differ depending on the type of television system.
2. Description of the Prior Art
Within the presently utilized television systems other than the NTSC system that is used in Japan and United States, are the PAL system used in Germany and the United Kingdom, as well as the SECAM system used in Eastern Europe. The audio intermediate frequencies of these systems may be classified into four types: 4.5 MHz (NTSC), 5.5 MHz (PAL), 6.0 MHz (PAL) and 6.5 MHz (PAL and SECAM).
In order to receive a plurality of television systems, television receivers must have a circuit that can automatically identify which of the above four audio intermediate frequencies a received frequency is. If not, the user must know the differences between the types of television systems and must change the frequencies manually.
A prior art example of this kind of circuit for automatic discrimination of audio intermediate frequencies can be found in Japanese laid-open patent application No. (Toku-Kai-Hei) 6-6691. In this conventional automatic discrimination circuit, the received signal is applied to four bandpass filter circuits, four detecting circuits and four A/D converters, each corresponding to one of the four audio intermediate frequencies. The resulting digital signal is then input into the CPU, which discriminates the most probable of the audio intermediate frequencies. This discrimination is performed with fuzzy logic, based on the input signal level (digital).
However, such a circuit for automatically discriminating intermediate frequencies with fuzzy logic must have a fuzzy logic program and a CPU (microprocessor) executing this program. Consequently, much cost and time for software development are needed, and since a comparatively processing-intensive CPU is needed, the overall cost of the circuit rises.
Furthermore, Japanese laid-open patent application No. (Toku-Kai-Hei) 6-6691 also describes the then prior art in simple terms. In this prior art, as several bandpass filter circuits were necessary to extract each individual audio intermediate frequency, there was a problem in the rise in cost and increase of circuit scale.
It is an object of the invention to solve these problems of the prior art and provide a circuit for automatic discrimination of audio intermediate frequencies with a comparatively simple circuit structure that can automatically discriminate received audio intermediate frequencies stably, while being compact and inexpensive.
A circuit for automatic discrimination of audio intermediate frequencies in accordance to this invention comprises logic circuits such as counter or decoders, and can automatically discriminate a frequency in a stable manner with a simple circuit.
A circuit for automatic discrimination of audio intermediate frequencies in accordance with the present invention comprises a high pass filter circuit for eliminating a synchronizing signal from an output of a video intermediate frequency detector; a synchronizing signal separation circuit for separating a synchronizing signal from the output of the video intermediate frequency detector; a waveform shaping circuit for shaping a waveform of an output signal of the high pass filter circuit; a counter for counting predetermined clock signals; a counter controlling circuit, which, triggered by the separated synchronizing signal, (i) causes the counter to count the number of the clock signals that are contained in a period of time that is defined based on a period of an output signal of the waveform shaping circuit, and (ii) causes the counter to output a counted value; and a discrimination signal output circuit for discriminating a range, within which the counted value falls, from a plurality of ranges, and outputting a discrimination signal according to the result of the discrimination, the plurality of ranges being partitioned corresponding to audio intermediate frequencies.
In the blanking period in which the synchronizing signal is contained, there is an audio signal, but no video signal (luminance signal and chrominance signal). Therefore, by controlling the counter by using the synchronizing signal as a trigger, a time period that is determined by the audio intermediate frequency can be accurately measured, without signal separation by a filter circuit. Thus, discrimination of the audio intermediate frequency can be performed from this measurement result.
It is preferable that the synchronizing signal used for triggering the counter control is a vertical synchronizing signal. It is also possible to use the horizontal synchronizing signal, but since the vertical blanking period is longer than the horizontal blanking period, the vertical blanking period leaves more time for discriminating the audio intermediate frequency.
It is preferable that the waveform shaping circuit comprises a frequency divider circuit. By counting the period or half-period of the frequency-divided audio intermediate signal with a counter, while using a clock with approximately the same frequency as the audio intermediate signal, adjacent audio intermediate frequencies can be discriminated with precision.
It is preferable that the discrimination signal output circuit comprises a plurality of comparators for comparing said counted value with a plurality of boundary values by which said plurality of ranges are patitioned; and a decoder for decoding an output of said plurality of comparators. Thus, a comparatively simple circuit for outputting a discrimination signal can be realized with regular circuit elements.
It is preferable that the circuit further comprises a latch for latching an output of the decoder. If the output of said decoder indicates that said audio intermediate frequency is not within a predetermined frequency range, then the output of the said latch maintains its output without changing it. In this case, it will be determined that the output is erroneous due the influence of large continuous noise and the operation is stabilized by maintaining the prior discrimination result.